1. Field of the Invention
This invention relates to optical communication systems and methods of operation for performance recovery by post-transmission dispersion compensation.
2. Background Discussion
One method available to increase the capacity of an optical communication system is Wavelength-Division Multiplexing (WDM). WDM makes use of the wideband property of an optical fiber and can increase the capacity of optical communication systems to several tens to several hundred of Gbit/s. In a WDM system each channel suffers chromatic dispersion from the transmission fiber. Chromatic dispersion results from the frequency dependence of the refractive index of silica as well as the waveguide contribution to the effective refractive index. Generally, chromatic dispersion tends to widen the pulses of the digital trains and, hence, creating intersymbol interferences.
Dispersion compensating fibers (DCF) have an opposite dispersion characteristic to the dispersion characteristic of most transmission optical fibers. In the prior art, dispersion-compensating optical fibers have been inserted into the optical transmission to prevent the increase in the quantity of chromatic dispersion for a specific wavelength. This type of dispersion compensating optical fiber has negative dispersion and the increase in the quantity of positive dispersion at a specific wavelength in an optical transmission fiber is reduced by using dispersion-compensating fibers. However, different channel can experience different levels of dispersion before and/or after dispersion compensation. The optimum adjustment of the dispersion levels to get the optimum performance is the main object of this new invention.
DCF can also enhance transmission performance by pre- or post-transmission compensation. Pre-compensation is positioning the DCF before the first transmission fiber of a transmission line. Post-compensation is positioning the DCF after the last transmission fiber of a transmission line.
Besides chromatic dispersion, another factor limiting transmission relates to nonlinear effects. Nonlinear effects induce Self-Phase Modulation (SPM) of the optical pulse. The instantaneous frequency diminishes at the start of the pulse and then increases at its end, proportionally to the derivative of the optical power. The nonlinear effects induce a widening of the spectrum in a spectral composition that fosters a substantial widening for negative chromatic dispersions.
What is needed in the art is an improved system and method for introducing dispersion compensating fibers into a WDM optical transmission system operating at high bit rates, e.g. 10 Gb/s to minimize nonlinear effects and chromatic dispersion.
Prior art related to adjustment of nonlinear and chromatic dispersion in optical communication systems includes the following:
U.S. Pat. No. 5,721,800 to T. Kato et al., issued Feb. 24, 1998 (Kato) discloses a dispersion shifted fiber having a structure for effectively lowering polarization mode dispersion. The dispersion shifted fiber is a single mode, optical fiber mainly composed of silica glass and has a xe2x80x9czeroxe2x80x9d dispersion wavelength set within the range of at least 1.4 xcexcm but no longer than 1.7 xcexcm.
U.S. Pat. No. 5,430,822 to M. Shigematsu et al., issued Jul. 4, 1995 (Shigematsu) discloses an optical communication system that compensates for the dispersion of an optical fiber serving as a transmission path. A dispersion compensation fiber having a sufficient length to compensate for chromatic dispersion of the optical fiber is divided into portions, each portion has a length selected so as to maintain the linear characteristics of a relative intensity noise of the dispersion compensating fiber. The divided portions of the dispersion compensating fiber are inserted in the path of the optical fiber while they are optically separated.
An article entitled xe2x80x9cImpact of Residual Dispersion on Self-Phased Modulation (SPM)Related Power Margins in 10 Gbit/s-based Systems Using Standard Single Mode Fibers (SMF)xe2x80x9d by G. Bellotti et al., published ECOC ""98, Sep. 20-24, 1998, Madrid, Spain, at pages 681-682 (Bellotti) discloses the most performing dispersion compensating technique for single mode fiber transmissions at 10 Gbit/s per second is post compensation. The publication provides a design rule for wave-division multiplexed self-phased modulation-limited transmissions.
U.S. Pat. No. 5,343,322 to F. Pirio et al., issued Aug. 30, 1994 (Pirio) discloses a transmitter and receiver stations connected by a monomode optical fiber with negative chromatic dispersion at the operating wavelength. The receiver station comprises devices to compensate for the distortion due to the nonlinear and chromatic dispersions. The compensation device carries out a positive chromatic dispersion of the received signal. The amplitude of the positive chromatic dispersion is a function of the amplitude of the negative chromatic dispersion induced by the optical fiber as well as of the mean on-line optical power of the signal transmitted on the optical fiber.
U.S. Pat. No. 5,854,871 to Y. Akasaka, issued Dec. 29, 1998 (Akasaka) discloses a wavelength division-multiplex transmission system constructed by connecting a dispersion-shifted optical fiber capable of zero dispersion wavelength to a positive dispersion sloped optical fiber. The fiber has a positive dispersion slope in the range from 1530 nanometers (nm) to 1560 nm and has almost the same dispersion as the same zero dispersion wavelength. The dispersion slope of the positive dispersion slope optical fiber is counterbalanced and compensated. The dispersion around a wavelength from 1530 nm to 1560 nm is made almost zero.
U.S. Pat. No. 5,361,319 to A. J. Antos et al., issued Nov. 1, 1994 (Antos) discloses a family of dispersion compensating optical fibers that are adapted for use with conventional single-mode transmission fibers that are optimized for zero dispersion compensation at a wavelength in the range of 1290 nm to 1330 nm. The compensating fibers are capable of providing dispersion more negative than xe2x88x9220 ps/nm-km and attenuation less than 1 dB/km at wavelengths in the 1520-1565 nm region. Certain other dispersion compensating fibers exhibit dispersion versus wavelength relationship having a negative slope in the 1520-1565 nm region to compensate for the dispersion versus wavelength slope of the transmission fiber. The dispersion compensating fiber can be advantageously combined with a fiber amplifier to form a compensator that is adapted to overcome attenuation introduced to the system by the dispersion compensating fiber.
None of the prior art discloses optimizing fiber optic communication system performance through post-transmission dispersion compensation using positive and negative compensation selected according to the residual dispersion slope of the fiber link on a per channel basis.
An object of the invention is an optical communication system and method of operation which overcomes dispersion and nonlinearities in wavelength division multiplex (WDM) transmissions.
Another object is an optical communication system and method of operation in WDM transmissions using post-compensation of dispersion to optimize system performance.
Another object is an optical communication system and method of operation using positive and negative dispersion compensation for optimized WDM performance.
Another object is an optical communication system and method of operation using post-dispersion compensation with selected positive and negative compensation.
These and other objects, features and advantages are achieved in an improved single-channel (SC) or Wavelength-Division Multiplexed (WDM) fiber optic system using post-transmission dispersion compensation. A SC/WDM transmitter (TX) is coupled to a SC/WDM receiver (RX) through transmission fibers including standard unshifted fibers (STD) and other transmission fiber types, e.g. dispersion-compensating fibers (DCFs). Amplifier units are installed at defined intervals referred to as amplifier spacing. Each unit includes first, second and third stage amplifiers. Dispersion-compensating fibers (DCFs) are typically installed between the first and second stage amplifiers. Optimum performance of the system (as measured by the minimum eye closure penalty for instance) depends on the length of DCF included in each amplifier. However, by using dispersion compensation at the end of the line, system performance improves significantly without changing the length of DCF in all amplifiers. Adjusting the dispersion at the end of the line is achieved by increasing or reducing the length of DCF in the last amplifier or by using any other dispersion compensating devices such as fiber Bragg gratings, etc. The adjustments of the dispersion compensation using positive and negative compensation at the end of the line can xe2x80x98curexe2x80x99 systems that would otherwise be considered unusable. The results hold even in presence of high power injected in the DCF. Post-transmission dispersion compensation applies to SC as well as for WDM systems. The method is especially efficient for system at bit rates of 10 Gb/s and above with the Non-Return-to-Zero (NRZ) format but applies to all modulation formats and types of transmission fibers.